Individuals III-3 and III-4 are expecting their first child when they become aware that they both have a family history of this recessive condition. As their genetic counselor, you can calculate the probability that they are carriers and that their child will be affected with the condition.

for III-3 it's 1/2 because his mother (represented by a circle) is Rr and his father (represented by a square) whose genotype isn't stated is also Rr. This is deduced from the fact that one of their offspring (III-1) is positive for the condition (which means it's rr). This means that the father who isn't positive for the condition is carrying an r gene making him Rr. When you do a punnet square and cross both parents the probability that III-3 will be a carrier (Rr), is 1/2.

for III-4 it's 1/2, because when you use a punnet square and cross both parents. there's a 1/2probability that III-4 is a carrier.

The probability that IV-1 will be affected (rr) is 1/8, because you multiply the probability that both parent's are carriers which is 1/2 * 1/2 = 1/4 then you multiply that by the probability that a cross between carriers will result in a homozygous recessive genotype (rr) which you can get by doing a punnet square of Rr x Rr which gives a 1/4 probability and 1/4*1/4 = 1/8

Note: The original answer of 2/3 for III-3 was correct because the probability that III-3 is a carrier is calculated based on the available options. A punnet square is not entirely correct because the person is showing the dominant phenotype. The punnet square shows 3 options that will give the dominant phenotype, 2 of which are Rr (carrier).

Note: The original answer of 2/3 for III-3 was correct because the probability that III-3 is a carrier is calculated based on the available options. A punnet square is not entirely correct because the person is showing the dominant phenotype. The punnet square shows 3 options that will give the dominant phenotype, 2 of which are Rr (carrier).

Note: The original answer of 2/3 for III-3 was correct because the probability that III-3 is a carrier is calculated based on the available options. A punnet square is not entirely correct because the person is showing the dominant phenotype. The punnet square shows 3 options that will give the dominant phenotype, 2 of which are Rr (carrier).

You can calculate probabilities in pedigrees by considering the requirements for a specific outcome. In this case, the outcome that individual IV-1 will be affected has four requirements:

1.Individual III-3 is a carrier (probability = 2/3).2.Individual III-4 is a carrier (probability = 1/2).3.Individual III-3 passes the r allele to his child (probability = 1/2, assuming III-3 is a carrier, which is accounted for in requirement 1).4.Individual III-4 passes the r allele to her child (probability = 1/2, assuming III-4 is a carrier, which is accounted for in requirement 2).

Because requirements 1 AND 2 AND 3 AND 4 must be met for the outcome in question (individual IV-1 to be affected), you calculate the answer by applying the multiplication rule (as implied by “AND”):The probability that IV-1 will be affected (rr) = 2/3 x 1/2 x 1/2 x 1/2 = 1/12.